When looking at any problem, we can't draw conclusions based on just one sentence, which can easily lead to misunderstandings, one-sided cognition of things themselves, and even completely distort the essence of things. We should understand the true meaning of the sentence in combination with the context before and after, especially the social background at the time, otherwise it is easy to take it out of context.
First of all, Albert Einstein said something like that".Gravitational force does not exist"Is that so?
It did say. Of course, if you have to raise the bar, "Did you hear Einstein say it with your own ears?" Then I have nothing to say.
However, just because Einstein said that "gravity does not exist", we should not completely "let ourselves go" and think that Newton's theory of gravitation is completely wrong, or even deny all the theories proposed by Newton.
Of course, this is not advisable, and we need to understand the context in which Einstein said the words "gravity does not exist", and why he said this.
Let's take a detailed look at the background of the times, starting with the Newton era.
We all know that more than 300 years ago, Newton put forward the great theory of gravitation on the "shoulders" of giants such as Kepler and Galileo, believing that gravity will arise between everything in the universe, and the magnitude of gravity is proportional to mass and inversely proportional to the square of distance.
Newton at the time thoughtGravity is a type of action at a distance, which can be instantaneous, that is to say, the propagation of gravity does not take any time and is instantaneous.
Newton's theory of gravitation can indeed explain the operation of all things in the universe very well. However, the theory of gravitation also has a fatal "weakness":How exactly is the ubiquitous gravitational pull transmitted?
According to the theory of gravitation, there is a gravitational force between two objects that are far apart, but how does the gravitational force between two objects be transmitted to each other across time and space?
Newton, of course, knew that his theory of gravitation was flawed, and he couldn't explain how gravity was transmitted, and how gravity came into being. But with the level of scientific knowledge in Newton's time, this question was a bit difficult for him. Newton is great, but no matter how great a man is, he cannot escape the constraints of the times, and there must be limitations in any era, and no one can break through these limitations.
And this question is left hundreds of years later to another equally great figure, and he is Albert Einstein.
Albert Einstein also thought about the origin of gravity and why there is such a force as gravity.
However, Einstein thought about the problem at a completely different level, not from the gravitational force itself, but by extensionThe properties of space-time.
In 1915, Albert Einstein proposed the great general theory of relativity, which defined the nature of gravity in terms of space-time curvature. That is to say,Gravitational attraction is really just a manifestation of the curvature of space-time, gravity is also not an over-distance effect. Of course, you can also think that gravity does not exist, it is just an external manifestation of the curvature of space-time.
According to the interpretation of general relativity, space-time itself is very elastic, and the movement of any object in space-time will have different degrees of influence on the surrounding space-time, just like if we throw a stone into a pond, the water in the pond will produce ripples or whirlpools, which will naturally affect the objects in the pond.
Moreover, any object, regardless of its mass, will have an impact on the surrounding space-time structure, but to varying degrees.
The perturbation of space-time by an object will naturally form"Time Trap" or "Time Vortex"., that is, space-time distortion, and this distortion of course also affects the surrounding objects, causing the surrounding objects to move in the direction of "space-time sinking or vortex", and this is gravity.
In other words, everything in the universe is actually doing it along a curved space-time."Geodesics"Motion, the so-called geodesic, is the shortest distance in four-dimensional space-time, and it is also the "straight" distance in four-dimensional space-time, but it is curved from a three-dimensional perspective.
However, only the curvature of space-time caused by massive objects will be more obvious, and the degree of curvature of space-time by objects with less mass is very weak and difficult to show.
When it comes to space-time bending, it is necessary to emphasize here that when describing space-time bending on the Internet, it will be described in the form of "spring bed bending downward", in fact, this way of description is not rigorous, because there is no concept of "up and down" in space, and space-time bending certainly does not know how to bend "down".
Actually,The complete direction of space-time is the direction towards the center of mass of the object, not "down". Of course, the schematic diagram on the Internet is more of a popular way to describe the curvature of space-time, which is more convenient for us to understand, but many times popular often means less rigorous.
So the question is, since the essence of gravity is the curvature of space-time, that is, gravity does not actually exist, why has Newton's theory of gravitation not been eliminated? Even now it is the theory of gravity that governs our lives, not the theory of relativity?
The reason is simple, although science is a very rigorous discipline, science is ultimately to serve human beings, which means that rigor is not what people value the mostPracticalityStronger is what people value.
Generally speaking, it doesn't matter if it's a black cat or a white cat, catching a mouse is a good cat.
Compared with the gravitational field equation of Einstein's general relativity system, Newton's theory of universal gravitation is actually an approximation, which is a special case and approximation of the gravitational field equation in the case of low gravity. That is to say, what gravitational force can explain, the gravitational field equation can also explain, and what gravitational force cannot explain, general relativity and gravitational field equations can also explain it, for example, the precession problem of Mercury, the theory of gravitation cannot explain it, but it can be explained very well by curvature of space-time.
But don't think that gravitational force as an approximation is useless: why use the less precise law of gravitation when there are more precise equations for gravitational fields?
As I just said, science has entered human life, and it is more practical, and the practicality of the theory of gravitation is much better than the bending of space-time. To put it bluntly, although gravitational force is only an approximation of the curvature of space-time, the error is negligible, and it will not have any impact on our daily life at all.
What's more, Newton's equation of gravitation is so simple that you can understand and calculate it at a junior high school math level.
On the other hand, the gravitational field equation under the general relativity system is a system of second-order nonlinear partial differential equations, and it is quite difficult to solve this equation, even Einstein himself made many predictions from the gravitational field equation through many approximation methods.
An analogy is used to understand the difference between gravitational force and gravitational field equations.
You bought a bag of 20 pounds of rice and want to move the rice to your home, your house is on the 6th floor and there is no elevator, what will you do?
Just 20 pounds of rice, any adult should choose to carry it to the 6th floor by himself, of course, you can also ask others to carry it, nothing more than spending some money, in short, you will choose to use someone to carry it directly.
But what if you use an oversized crane to lift that bag of rice directly to the 6th floor?
Theoretically, of course, it is possible, but this involves the question of practicality: who would carry a 20-kilogram bag of rice in a crane in reality?
Choosing gravitational force is like choosing to use a person on his back, and the gravitational field equation is equivalent to that huge crane. Using a crane looks more effortless, but in fact it is less feasible and will bring more trouble, which is completely unnecessary.
This is why it is not the general theory of relativity, the curvature of space-time, but the law of gravitation proposed by Newton that governs our daily lives today.
For example, rocket launches, various satellites and probes launches, the theory used is the theory of gravitation, which is accurate enough for the launch of Martian satellites. That insignificant error is not even as big as the impact of your sneeze on the rocket launch!
Of course, this is not to say that Einstein's general theory of relativity is useless, but it just means that in the small space of our earth, the "hero" of general relativity is not used at all, and its use is thereSpace is the vast universe。Mankind will eventually leave the earth and fly into the vastness of space, and that day will be the time when general relativity will reign.
The most typical use of the theory of relativity is the satellite positioning system. Since navigation satellites are usually in space more than 20,000 kilometers above the ground, the speed can reach 140,000 km.
In the special theory of relativity, the faster the speed, the slower the time, and the time on a satellite is 7 microseconds slower per day than the time on the ground.
From the perspective of general relativity, the gravitational pull of the satellites in the sky is smaller, and the time will be faster, about 45 microseconds faster than the ground time per day.
Combined, the time of the satellite is faster than the time on the ground every day38 microsecondsAround. One second is equivalent to one million microseconds, and 38 microseconds is a very short time, but don't underestimate such a short time, if you don't correct the time on the satellite, the accumulated error every day is likely to let the navigation system navigate you directly to the ditch!
Summary
At this point, we should understand why Einstein said that "gravity does not exist", Einstein just described the nature of gravity, so that we can understand the laws of the world more accurately.
In fact, space-time bending may not be the most accurate, the interpretation of space-time bending is not the ultimate truth, and Einstein did not explain why space-time bends, and this question is naturally left to the next great physicist!
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